GLACIATION OF THE YAKUTAT BAY REGION 201
It is noteworthy that, while smaller glaciers are often covered with broad sheets of ablation moraine, through glaciers are not so characterized. In the active tidal glaciers, such as the Hubbard and Nunatak, this might be explained by the failure of ablation in connection with iceberg discharge; but in such glaciers as the Hidden and Fourth, where ablation lowers the terminus to a thin edge, and where the terminus lies in the same relation to sea level as smaller, neighboring moraine-covered glaciers, this explanation fails. The absence of moraine sheets in the lower glacier in such cases must be explained by failure of debris to be incorporated in the ice so extensively in the through glaciers as in the smaller valley glaciers. This lack of incorporated debris may be due to a combination of several causes, varying in relative importance in the different glaciers, among which are the following:—(a) the fact that a considerable part of the ice is supplied by snow falling directly on the glacier; (b) the breadth of the through glaciers, making it impossible for avalanches to spread completely across the glaciers as they may in narrower glaciers; (c) the relative weakness of tributaries by which the debris that they contribute is kept on one side of the main glacier by the greater force of its current; (d) the deep snows which mantle so large a proportion of the upper mountain slopes with so deep a cover that the mountain rocks are protected far more than the steep, bare slopes which border a large portion of the smaller valley glaciers.
Even where through glaciers emerge beyond the mountain valleys and expand in piedmont areas, as in the Malaspina Glacier, some of whose tributaries are probably through glaciers, the extent of the sheet of ablation moraine is proportionately less than that of the smaller glaciers. Thus, although there are such broad sheets of ablation moraine on the Malaspina as to have attracted wide attention, the most extensive of these are really marginal, and related to the lateral accumulations by the tributaries which supply the Malaspina ice. Extensive as these sheets of ablation moraine are on the Malaspina Glacier, when compared to the glacier system as a whole they are far less notable than the ablation moraine of the Atrevida-Lucia piedmont glacier system. The Malaspina and its tributaries are dominantly clear ice, with only relatively-small, peripheral, ablation moraine areas and larger lateral sheets; but the smaller Atrevida-Lucia piedmont glacier is completely moraine-covered, and the morainic sheet even spreads well up the valley portion of each tributary. Extensive morainic sheets are, therefore, not characteristic of the through glaciers, but rather of the smaller glaciers, being present even where they terminate within their mountain valleys, but being far more developed where their termini expand in piedmont areas outside of their mountain valleys.
The Glacier Termini. It is the lower portions of the Alaskan glaciers that have so-far been most carefully studied, and it is these portions, also, that present the most varied and interesting phenomena, and those which supply most facts of value for comparison with glaciers of other regions, both existing and extinct. Among the Alaskan glaciers there are many different conditions in the termini, and as most, if not all of these are illustrated in the Yakutat Bay region, it seems worth while to briefly state in the following paragraphs some of the more notable differences observed.
The termini of many glaciers, probably the majority, are at their junction with larger glaciers to which they are tributary. The conditions here vary greatly according to the relative size of tributary and main glacier and to the position and direction at which they enter. In the vast majority of cases these tributaries cease at once to have notice-